There are various flavors of Marathon tires. The Marathon Plus has the greatest puncture protection, but the penalty is a sluggish ride because of the heavy weight of the tire.

If your tire has only 300 miles on it, I would continue using it to see how it goes. The puncture could have been nothing more than a fluke. Another thing you can try is using a sealant, like Orange Seal on your tubes. This is made for tubeless tires, but it also works on tubes. It won't prevent punctures, but it allows you to pull out whatever punctured your tire, re-inflate, and theoretically continue riding without having to change the tube. It doesn't work with every type of puncture, but it helps. If you continue getting punctures, at that point it makes sense to consider the Marathon Plus.